Coaxial radio frequency adapter and method

ABSTRACT

A coaxial radio frequency adapter and method are disclosed. An adapter has a tapered signal pin and a tapered ground sleeve to maintain a consistent impedance and minimize reflections while connecting two elements having different dimensions. A method employs an adapter to characterize losses in a system for evaluating a device under test.

BACKGROUND

[0001] There are many environments which require that a transmissionloss be small, characterizable, and/or predictable. For example, in asystem for performing a production test on a radio frequency (RF) deviceunder test (DUT), an RF test probe may be used to contact the DUT. TheDUT may for example be an integrated circuit board for a wirelesscommunication device.

[0002] As shown in FIG. 1, DUT 2 may be placed in RF test fixture 4. RFprobe 6 may be mounted in RF test fixture 4 in a position to contact DUT2. RF probe 6 may be connected by coaxial cable 5 and coaxial cable 3 toRF equipment rack 9. RF equipment rack 9 may include a signal generator11, a switch 13, a spectrum analyzer 15, and a test controller PC 17. InFIG. 1, coaxial cable 5 is shown in electrical communication with switch13 through coaxial cable 3. Test controller PC 17 may connect coaxialcable 5 through switch 13 to spectrum analyzer 15 in order to measurethe RF transmitting power of DUT 2. Alternatively, test controller PC 17may connect coaxial cable 5 through switch 13 to signal generator 11 inorder to measure the RF receiving sensitivity of DUT 2.

[0003] Such measurements may be made during production runs of DUT 2 forquality control purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004]FIG. 1 illustrates a production test configuration used toevaluate a device under test.

[0005]FIG. 2 is an exploded view of an adapter in relation to a radiofrequency probe and a coaxial cable.

[0006]FIG. 3 is an exploded close-up view of the adapter of FIG. 2.

[0007]FIG. 4 is a close-up top view of the adapter of FIG. 2 inunexploded form.

[0008]FIG. 5 is a close-up bottom view of the adapter of FIG. 2 inunexploded form.

[0009]FIG. 6 is a cross-section view of the adapter of FIG. 2 inpress-contact with a radio frequency probe.

[0010]FIG. 7 is a cross-section view of the adapter of FIG. 2 inpress-contact with a radio frequency probe and in screw engagement withthe sub-miniature assembly connector of a coaxial cable.

[0011]FIG. 7a is an enlarged view of part of FIG. 7.

[0012]FIG. 8 is a top view of the adapter mounted onto a fixturingblock.

[0013]FIG. 9 is a bottom view of the assembly of FIG. 8.

[0014]FIG. 10 shows the assembly of FIG. 8 in contact with an RF testfixture, an RF test probe, and a coaxial cable.

[0015]FIG. 11 illustrates a calibration configuration.

[0016]FIG. 12 illustrates a testing configuration for characterizinglosses in a system with a test fixture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0017]FIG. 1 illustrates a production test configuration used toevaluate DUT 2. Because the production test configuration may be used tomeasure RF characteristics associated with DUT 2, the inventors desireto know the extent of RF signal loss associated with the test equipmentitself.

[0018] A vector network analyzer can be used to evaluate RF signal lossthrough a closed circuit. For example, a closed circuit including RFtest probe 6 but not including any device under test could be evaluated.Typically, RF test probe 6 can be easily connected at one end to acoaxial cable. The other end (the “contact end”) of RF test probe 6,however, is meant for press-contact with DUT 2 and cannot easily beconnected to a coaxial cable. For example, the contact end may be springloaded so that the test probe is urged against DUT 2. The contact endmay have a sharp crown edge. An adapter of the present invention may beused to connect the contact end of RF test probe 6 to a sub-miniatureassembly (SMA) connector of a coaxial cable.

[0019]FIG. 2 is an exploded view of adapter 7 in relation to RF probe 6and coaxial cable 8. Adapter 7 is devised to electrically connect RFprobe 6 to coaxial cable 8 with no change in impedance and noreflections. Adapter 7 includes adapter ground sleeve 14 and adaptersignal pin 12. Adapter signal pin 12 fits inside of adapter groundsleeve 14. Adapter signal pin 12 may be spaced apart from adapter groundsleeve 14 by a dielectric material surrounding at least part of adaptersignal pin 12. Adapter ground sleeve 14 may be made of copper or otherconductive material that is softer than the contact end of RF test probe6 in order to facilitate a good press-connection between adapter groundsleeve 14 and RF test probe 6.

[0020] Coaxial cable 8 has SMA connector 10 including barrel nut 19.Barrel nut 19 serves as the ground lead of coaxial cable 8. Adapterground sleeve 14 has screw threads 25 adapted to engage barrel nut 19 ofcoaxial cable 8.

[0021]FIG. 3 is an exploded close-up view of adapter 7. Adapter groundsleeve 14 has mounting flange 18. Adapter signal pin 12 has a probecontact end 20 and a cable contact end 21. At cable contact end 21,adapter signal pin 12 has hole 16 for receiving connector signal pin 30(FIG. 7) from SMA connector 10 of coaxial cable 8. Connector signal pin30 serves as the signal lead of coaxial cable 8.

[0022]FIG. 4 is a top view of adapter 7. Adapter signal pin 12 is showninside of adapter ground sleeve 14. Mounting flange 18 has screw holes19.

[0023]FIG. 5 is a bottom view of adapter 7. Adapter signal pin 12 isshown inside of adapter ground sleeve 14.

[0024]FIG. 6 is a cross-section view of adapter 7 in press-contact withRF probe 6. RF probe 6 has RF probe signal pin 27 and RF probe groundsleeve 29. RF probe signal pin 27 contacts adapter signal pin 12; RFprobe ground sleeve 29 contacts adapter ground sleeve 14. RF probesignal pin 27 acts as a signal probe and RF ground sleeve 29 acts as aground probe.

[0025]FIG. 7 is a cross-section view of adapter 7 in press-contact withRF probe 6 and in screw engagement with SMA connector 10 of coaxialcable 8. SMA connector 10 has connector signal pin 30. Connector signalpin 30 is received in hole 16 at cable contact end 21 of adapter signalpin 12.

[0026]FIG. 7a is a magnified view of part of FIG. 7. Adapter signal pin12 is characterized by an inner radius r_(i). This is the distance fromthe center of adapter signal pin 12 to the outside surface of adaptersignal pin 12. Ground sleeve 14 is characterized by outer radius r_(o).This is the distance from the center of ground sleeve 14 to the innersurface of ground sleeve 14.

[0027] The design of adapter 7 may be adjusted to achieve a desiredimpedance according to the formula:$Z_{o} = \frac{{\ln \left( {r_{o}/r_{i}} \right)}\sqrt{\mu/ɛ}}{2\quad \pi}$

[0028] where μ=relative magnetic permeability of the conductor materialof the adapter signal pin and the adapter ground sleeve;

[0029] ε=relative permittivity of the dielectric material;

[0030] r_(i)=signal pin radius;

[0031] r_(o)=radius of the inside surface of the ground sleeve.

[0032] For example, a dielectric material surrounding adapter signal pin12 may be selected having a particular dielectric constant. As anotherexample, the ratio r_(o)/r_(i) can be adjusted. As a third example, thematerial of adapter signal pin 12 and adapter ground sleeve 14 may beadjusted according to the same formula. Any combination of those threeexemplary adjustments may be made to optimize the impedance equation fora particular application. In particular applications, adapter 7 may bedesigned to have an impedance of 50 ohms for use in a wirelesscommunication environment, or an impedance of 75 ohms for use in atelevision environment, for example.

[0033] The sides of SMA connector 10 and/or ground sleeve 14 may haveflats to facilitate assembly using a torque wrench, which createsconsistent tightness and repeatable signal loss through the connections.

[0034] RF probe signal pin 27 is characterized by probe pin radiusr_(p). This is the distance from the center of RF probe signal pin 27 tothe outer surface of RF probe signal pin 27. RF probe ground sleeve 29is characterized by probe sleeve radius r_(s). This is the distance fromthe center of RF probe ground sleeve 29 the inside surface of RF probeground sleeve 29. In a preferred embodiment shown in FIG. 7a, at thepoint where adapter 7 contacts RF probe 6, probe pin radius r_(p) isequal to inner radius r_(i) and probe sleeve radius r_(s) is equal toouter radius r_(o).

[0035] In an exemplary application, probe pin radius r_(p) is smallerthan the radius of connector signal pin 30 and probe sleeve radius r_(s)is smaller than the radius of the inside of barrel nut 19. In such asituation, as shown in FIG. 7, adapter 7 provides a gradually taperedtransition from the large dimensions of SMA connector 10 to the smallerdimensions of RF test probe 6 without any abrupt steps that could createreflections, signal loss, or parasitic capacitances. The taper design ofadapter 7 can be changed to create smooth impedance-matched transitionsbetween various size connectors.

[0036] Preferably, adapter 7 has the same impedance as RF probe 6 andcoaxial cable 8. Maintaining the ratio r_(o)/r_(i) along the length ofadapter 7, from probe contact end 20 to cable contact end 21, can ensurea consistent impedance throughout adapter 7. Thus, there is provided anadapter that connects two different size components and minimizes signalloss, reflection, and parasitic capacitances.

[0037]FIG. 8 is a top view of adapter 7 mounted with screws 42 ontofixturing block 40.

[0038]FIG. 9 is a bottom view of the assembly of FIG. 8. Registrationsurfaces 44 line the underside of fixturing block 40.

[0039]FIG. 10 shows the assembly of FIG. 8 in contact with RF testfixture 4, RF test probe 6, and coaxial cable 8. RF test probe 6 ismounted on RF test fixture 4. Registration surfaces 44 of fixturingblock 40 line up with elements (not shown) on RF test fixture 4 in orderto accurately place adapter 7 in axial alignment with RF test probe 6.This arrangement exemplifies a use for adapter 7 to characterize lossesassociated with the test equipment of FIG. 1.

[0040]FIG. 11 illustrates a calibration configuration in which vectornetwork analyzer 50 is used to characterize losses in a system without atest fixture. Vector network analyzer 50 has RF out port 52 and RF inport 54. One end of coaxial cable 3 is connected to RF out port 52. Theother end has an SMA connector which contacts adapter 7 in fixturingblock 40. Adapter 7 is connected to coaxial cable 8, which in turn isconnected to RF in port 54.

[0041]FIG. 12 illustrates a testing configuration in which vectornetwork analyzer 50 is used to characterize losses in a system having atest fixture. Coaxial cable 3 is connected to RF out port 52 and isconnected to coaxial cable 5. Thus, coaxial cable 5 is in electricalcommunication with RF out port 52 through coaxial cable 3. Coaxial cable5 is fixed in test fixture 4 and is connected to RF test probe 6. RFtest probe 6 is fixed in test fixture 4 and is shown in press contactwith adapter 7. The position of adapter 7 is fixed by fixturing plate 40such that adapter 7 is in axial alignment with RF test probe 6. Adapter7 is connected to SMA connector 10 of coaxial cable 8. Coaxial cable 8is connected to RF in port 54.

[0042] Taken together, FIG. 11, FIG. 12, and FIG. 1 can be used toillustrate a method exemplifying the present invention. The methodcomprises the following.

[0043] First, as shown in FIG. 11, coaxial cable 3, adapter 7, andcoaxial cable 8, all having the same impedance, are connected in seriesto form a calibration configuration in which adapter 7 contacts an SMAconnector of coaxial cable 3. Second, a first radio frequency signal issent through the calibration configuration illustrated in FIG. 11.Third, a first loss is measured in the first radio frequency signalafter the first radio frequency signal is sent through the calibrationconfiguration of FIG. 11.

[0044] Fourth, as shown in FIG. 12, coaxial cable 5 and radio frequencytest probe 6 are fixed in test fixture 4. Fifth, coaxial cable 3,coaxial cable 5 and radio frequency test probe 6 in test fixture 4,adapter 7, and coaxial cable 8 are connected in series to form a testconfiguration in which adapter 7 contacts radio frequency test probe 6.Sixth, a second radio frequency signal is sent through the testconfiguration shown in FIG. 12. Seventh, a second loss is measured inthe second radio frequency signal after the second radio frequencysignal is sent through the test configuration of FIG. 12.

[0045] Eighth, the first loss is subtracted from the second loss toderive a fixture loss. Knowing the fixture loss is useful when analyzingtest readings for DUT 2 as described above with reference to FIG. 1.Accurate characterization of loss through the test equipment ensuresthat the measured RF characteristics of DUT 2 are not affected bylosses. That is, the losses associated with coaxial cable 5, coaxialcable 3, and radio frequency test probe 6 in test fixture 4, are addedback to the transmission and receive measurements of DUT 2 to determinethe true (corrected) performance of the DUT itself.

[0046] The fourth through eighth steps may be repeated for other testfixtures. In other words, the first through fourth steps can beperformed one time in order to characterize the calibration equipmentconsisting of coaxial cable 3, adapter 7, and coaxial cable 8. Thecalibration equipment may then be used to characterize accurately eachof a plurality of test fixtures where each of the test fixtures has itsown RF test probe fixed thereto. Once the RF signal loss attributable tothe calibration equipment is known, the calibration equipment can beused repeatedly to determine the RF signal loss in any of a number offixtures. Such fixtures can then be used to evaluate RF characteristicsassociated with DUTs. One such fixture is shown in FIG. 1.

[0047] The above disclosure and drawings merely illustrate the inventiveconcepts. The skilled artisan will recognize therefrom that manyvariations and permutations can be made without departing from thespirit of the invention. The exemplary embodiments disclosed herein arenot meant to limit the scope of the invention.

What is claimed is:
 1. A coaxial adapter comprising: a ground sleeve having a first ground sleeve end adapted to contact a ground lead of a coaxial cable and a second ground sleeve end adapted to contact a ground probe of a test probe, the ground sleeve being characterized by a first outer radius at said first ground sleeve end and a second outer radius at said second ground sleeve end; and a signal pin positioned inside of and spaced apart from the ground sleeve, the signal pin having a first signal pin end adapted to contact a signal lead of a coaxial cable and a second signal pin end adapted to contact a signal probe of a test probe, the signal pin being characterized by a first inner radius at said first signal pin end and a second inner radius at said second signal pin end; wherein the first outer radius is different than the second outer radius, the first inner radius is different than the second inner radius, and a ratio of the first inner radius to the first outer radius is the same as the ratio of the second inner radius to the second outer radius.
 2. The coaxial adapter of claim 1 wherein the signal pin is externally tapered and the ground sleeve is internally tapered to maintain said ratio constant from the first signal pin end to the second signal pin end.
 3. The coaxial adapter of claim 2 wherein the ground sleeve is made of copper.
 4. The coaxial adapter of claim 2 wherein the first ground sleeve end is externally threaded.
 5. The coaxial adapter of claim 4 wherein the second ground sleeve end is externally unthreaded.
 6. A testing system comprising: a network analyzer having a radio frequency out port and a radio frequency in port; a first coaxial cable having a first end connected to the radio frequency out port and a second end; a radio frequency test probe having a first end electrically coupled to the second end of the first coaxial cable and a second end; an adapter having a first end in contact with the second end of the radio frequency test probe and a second end; and a second coaxial cable having a first end connected to the second end of the adapter and a second end in communication with the radio frequency out port; wherein the first coaxial cable, the radio frequency test probe, the adapter, and the second coaxial cable all have the same impedance.
 7. The testing system of claim 6 wherein the adapter comprises: a ground sleeve having a first ground sleeve end adapted to contact a ground lead of a coaxial cable and a second ground sleeve end adapted to contact a ground probe of the test probe; and a signal pin positioned inside of and spaced apart from the ground sleeve, the signal pin having a first signal pin end adapted to contact a signal lead of a coaxial cable and a second signal pin end adapted to contact a signal probe of the test probe.
 8. The testing system of claim 7 wherein: the ground sleeve is characterized by a first outer radius at said first ground sleeve end and a second outer radius at said second ground sleeve end; the first outer radius is different than the second outer radius; the signal pin is characterized by a first inner radius at said first signal pin end and a second inner radius at said second signal pin end; the first inner radius is different than the second inner radius; and a ratio of the first inner radius to the first outer radius is the same as the ratio of the second inner radius to the second outer radius.
 9. The testing system of claim 8 wherein the signal pin and the ground sleeve are both tapered to maintain said ratio constant throughout the adapter.
 10. The testing system of claim 9 wherein the signal probe is characterized by a signal probe radius equal to the second inner radius of the signal pin and the ground probe is characterized by a ground probe radius equal to the second outer radius of the ground sleeve.
 11. The testing system of claim 7 further comprising an adapter fixturing plate, the adapter being secured to the adapter fixturing plate and the adapter fixturing plate have registration surfaces to position the adapter with respect to the radio frequency probe.
 12. A method comprising: connecting in series at least a first coaxial cable, an adapter, and a second coaxial cable, all having the same impedance, to form a calibration configuration; sending a first radio frequency signal through the calibration configuration; measuring a first loss in the first radio frequency signal after the first radio frequency signal is sent through the calibration configuration; placing a radio frequency test probe in a test fixture; connecting in series at least the first coaxial cable, the radio frequency test probe in the test fixture, the adapter, and the second coaxial cable to form a test configuration in which the adapter contacts the radio frequency test probe; sending a second radio frequency signal through the test configuration; measuring a second loss in the second radio frequency signal after the second radio frequency signal is sent through the test configuration; and subtracting the first loss from the second loss to derive a fixture loss.
 13. The method of claim 12 further comprising; contacting a device under test with the radio frequency test probe in the test fixture.
 14. The method of claim 13 wherein the calibration configuration includes no wireless component, the test configuration includes no wireless component, and the device under test is a wireless component.
 15. The method of claim 12 wherein the adapter comprises: a ground sleeve having a first ground sleeve end adapted to contact a ground lead of a coaxial cable and a second ground sleeve end adapted to contact a ground probe of the test probe; and a signal pin positioned inside of and spaced apart from the ground sleeve, the signal pin having a first signal pin end adapted to contact a signal lead of a coaxial cable and a second signal pin end adapted to contact a signal probe of the test probe.
 16. The method of claim 15 wherein: the ground sleeve is characterized by a first outer radius at said first ground sleeve end and a second outer radius at said second ground sleeve end; the first outer radius is different than the second outer radius; the signal pin is characterized by a first inner radius at said first signal pin end and a second inner radius at said second signal pin end; the first inner radius is different than the second inner radius; and a ratio of the first inner radius to the first outer radius is the same as the ratio of the second inner radius to the second outer radius.
 17. The method of claim 16 wherein the signal pin and the ground sleeve are both tapered to maintain said ratio constant throughout the adapter.
 18. A coaxial adapter comprising: a ground sleeve having a first ground sleeve end adapted to contact a ground lead of a coaxial cable and a second ground sleeve end adapted to contact a ground probe of a test probe, the ground sleeve being characterized by a radius r_(o)measured from the center of the ground sleeve to the inner surface of the ground sleeve; a signal pin positioned inside of and spaced apart from the ground sleeve, the signal pin having a first signal pin end adapted to contact a signal lead of a coaxial cable and a second signal pin end adapted to contact a signal probe of a test probe, the signal pin being characterized by a radius r_(i) measured from the center of the test probe to an outer surface of the test probe, and the signal pin and ground sleeve having a relative magnetic permeability μ; and a dielectric material interposed between at least part of the signal pin and at least part of the ground sleeve, the dielectric material having a relative permittivity ε; wherein r_(o), μ, r_(i), and ε are selected such that an impedance Z_(o) of the adapter matches an impedance of the test probe according to the following formula: $Z_{o} = \frac{{\ln \left( {r_{o}/r_{i}} \right)}\sqrt{\mu/ɛ}}{2\quad \pi}$


19. The coaxial adapter of claim 18 wherein the dielectric material is air.
 20. The coaxial adapter of claim 18 wherein the signal pin is tapered from the first signal pin end to the second signal pin end.
 21. The coaxial adapter of claim 19 wherein the ground sleeve is tapered from the first ground sleeve end to the second ground sleeve end.
 22. The coaxial adapter of claim 21 wherein a ratio r_(o)/r_(i) is maintained constant over a length of said coaxial adapter. 